bpo-30190: improved error msg for assertAlmostEqual(delta=...) (#1331)

* #30190 / unittest / assertAlmostEqual(delta=...) / error msg: show the difference between the 2 numbers in case of failure

* safe_repr() diff

* also show difference when passing 'places' argument

* refactoring

* update Misc/NEWS

Lib/unittest/case.py

@@ -856,23 +856,28 @@ class TestCase(object):
         if delta is not None and places is not None:
             raise TypeError("specify delta or places not both")
 
+        diff = abs(first - second)
         if delta is not None:
-            if abs(first - second) <= delta:
+            if diff <= delta:
                 return
 
-            standardMsg = '%s != %s within %s delta' % (safe_repr(first),
-                                                        safe_repr(second),
-                                                        safe_repr(delta))
+            standardMsg = '%s != %s within %s delta (%s difference)' % (
+                safe_repr(first),
+                safe_repr(second),
+                safe_repr(delta),
+                safe_repr(diff))
         else:
             if places is None:
                 places = 7
 
-            if round(abs(second-first), places) == 0:
+            if round(diff, places) == 0:
                 return
 
-            standardMsg = '%s != %s within %r places' % (safe_repr(first),
-                                                          safe_repr(second),
-                                                          places)
+            standardMsg = '%s != %s within %r places (%s difference)' % (
+                safe_repr(first),
+                safe_repr(second),
+                places,
+                safe_repr(diff))
         msg = self._formatMessage(msg, standardMsg)
         raise self.failureException(msg)
 
@@ -890,16 +895,19 @@ class TestCase(object):
         """
         if delta is not None and places is not None:
             raise TypeError("specify delta or places not both")
+        diff = abs(first - second)
         if delta is not None:
-            if not (first == second) and abs(first - second) > delta:
+            if not (first == second) and diff > delta:
                 return
-            standardMsg = '%s == %s within %s delta' % (safe_repr(first),
-                                                        safe_repr(second),
-                                                        safe_repr(delta))
+            standardMsg = '%s == %s within %s delta (%s difference)' % (
+                safe_repr(first),
+                safe_repr(second),
+                safe_repr(delta),
+                safe_repr(diff))
         else:
             if places is None:
                 places = 7
-            if not (first == second) and round(abs(second-first), places) != 0:
+            if not (first == second) and round(diff, places) != 0:
                 return
             standardMsg = '%s == %s within %r places' % (safe_repr(first),
                                                          safe_repr(second),
@@ -908,7 +916,6 @@ class TestCase(object):
         msg = self._formatMessage(msg, standardMsg)
         raise self.failureException(msg)
 
-
     def assertSequenceEqual(self, seq1, seq2, msg=None, seq_type=None):
         """An equality assertion for ordered sequences (like lists and tuples).
 

Lib/unittest/test/test_assertions.py

@@ -223,9 +223,11 @@ class TestLongMessage(unittest.TestCase):
                              "^1 == 1 : oops$"])
 
     def testAlmostEqual(self):
-        self.assertMessages('assertAlmostEqual', (1, 2),
-                            ["^1 != 2 within 7 places$", "^oops$",
-                             "^1 != 2 within 7 places$", "^1 != 2 within 7 places : oops$"])
+        self.assertMessages(
+            'assertAlmostEqual', (1, 2),
+            ["^1 != 2 within 7 places \(1 difference\)$", "^oops$",
+             "^1 != 2 within 7 places \(1 difference\)$",
+             "^1 != 2 within 7 places \(1 difference\) : oops$"])
 
     def testNotAlmostEqual(self):
         self.assertMessages('assertNotAlmostEqual', (1, 1),

Misc/NEWS

@@ -10,7 +10,7 @@ What's New in Python 3.7.0 alpha 1?
 Core and Builtins
 -----------------
 
-- bpo-12414: sys.getsizeof() on a code object now returns the sizes 
+- bpo-12414: sys.getsizeof() on a code object now returns the sizes
   which includes the code struct and sizes of objects which it references.
   Patch by Dong-hee Na.
 
@@ -317,6 +317,10 @@ Extension Modules
 Library
 -------
 
+- bpo-30190: unittest's assertAlmostEqual and assertNotAlmostEqual provide a
+  better message in case of failure which includes the difference between
+  left and right arguments.  (patch by Giampaolo Rodola')
+
 - bpo-30101: Add support for curses.A_ITALIC.
 
 - bpo-29822: inspect.isabstract() now works during __init_subclass__.  Patch